In recent years, electric vehicles and hybrid vehicles mounted with an AC (alternating current) motor as a drive power source for the vehicle receives attention in light of social demands for improved fuel economy and reduced exhaust emissions. For example, some hybrid vehicles are so configured that a DC (direct current) electric power supply such as a secondary battery and an AC motor are connected with each other through a power conversion device such as an inverter. A DC voltage of the DC power supply is converted into an AC voltage by the inverter to drive the AC motor. The AC motor is controlled by a control apparatus in a manner disclosed in, for example, the following patent documents.    Patent document 1: JP-A-2004-159391    Patent document 2: JP-A-2010-124544    Patent document 3: JP-A-2008-86139
With respect to a control apparatus for an AC motor mounted in such a hybrid vehicle or an electric vehicle, it is known that a current sensor for detecting a phase current is provided only in one phase and not provided in other phases. The number of current sensors, the size of the vicinity of the three phase output terminals of the inverter, and the cost of a control system for the AC motor are thereby reduced. For example, a current estimation value in the phases (for example, U-phase and V-phase) other than a sensor phase (W-phase) is calculated based on a current detection value in one “sensor phase” (for example, W-phase) detected with a current sensor, a d-axis current command value and a q-axis current command value, and an electrical angle of the AC motor (for example, patent document 1 and 3).
As a control mode for an AC motor, a rectangular wave control mode is proposed (for example, patent document 2). In this control mode, a voltage utilization factor can be enhanced more than in a sine wave PWM control mode or overmodulation PWM control mode. Rectangular wave refers to a waveform, in which one cycle of current corresponds to one pulse.
The sine wave PWM control mode and overmodulation PWM control mode are equivalent to a current feedback control scheme. In this scheme, feedback is performed so that a d-axis current and a q-axis current calculated based on a detected value from a current sensor follow a d-axis current command value and a q-axis current command value. Meanwhile, in the rectangular wave control mode, the phase of a rectangular wave voltage is controlled by taking the following measure: an estimated torque is calculated from a d-axis current and a q-axis current calculated based on a detected value from a current sensor and feedback is performed so that this estimated torque follows a torque command value. This is thus equivalent to “torque feedback control scheme.”
The rectangular wave control mode is employed especially in a high rotation operation range, in which weak field control is required for suppressing counter electromotive force. In this mode, an absolute value of a weak field current as a negative d-axis current is minimized. Further, it is possible to minimize the number of times of switching of an inverter to reduce switching loss. Because of these characteristics, it is effective to switch the control scheme in drive control for a motor-generator or the like of a hybrid vehicle or an electric vehicle based on the operating state, such as number of rotations and output torque, of a motor, that is, the voltage required for driving the motor. For example, the sine wave control mode is used in a low rotation range, the rectangular wave control mode is used in a high rotation range and the overmodulation control mode is used in an intermediate rotation range between the low rotation range and the high rotation range.
In AC motors mounted in hybrid vehicles or electric vehicles, the number of rotations and output torque vary more widely than in AC motors in other technical fields. Therefore, it is desired to ensure that the current feedback control scheme and the torque feedback control scheme can be used together. Therefore, it is desired to use the current feedback control scheme and the torque feedback control scheme together.